Cell lining system



v. l.. BULLOUGH 3,457,158

CELL LINING SYSTEM l Filed Oct.

July 22, 1969 ATTORNEYJ' United States Patent O 3,457,158 CELL LININGSYSTEM Vaughn L. Bullough, Florence, Ala., assignor to Reynolds MetalsCompany, Richmond, Va., a corporation of Delaware v Filed Oct. 2, 1964,Ser. No. 401,067 Int. Cl. C22d 3/12, 3/02 U.S. Cl. 204-243 9 ClaimsABSTRACT OF THE DISCLOSURE This invention relates to electrolytic cellsfor the production of aluminum and other metals and to a novel methodfor controlling the thermal expansion of cell insulating materials.lMore particularly, the invention concerns a novel method forcontrolling the thermal expansion of powdered alumina.

Aluminum metal is conveniently produced in electrolytic cells by passinga current through a bath of molten cryolite containing dissolved aluminain a large tank lined with carbon which serves las one electrode, namelythe cathode. Large carbon blocks presented at the top of the bathfunction as the anode. Molten aluminum metal at a temperature of about1800" F. collects to the bottom of the cell `and is siphoned therefromperiodically. The cells may be lined with rammed rcarbon and may have arow of horizontal graphite bars extending from one side of the cellwhich serve to contact the pad of molten aluminum which collects at thebottom of the cell when in operation, thus acting as cathode currentcollectors. In newer types of construction of electrolytic reductioncells, refractory linings made of alumina and cryolite are employed, `asillustrated, for example, in U.S. Patent 3,093,570, and in copendingapplication Ser. No. 222,079, now U.S. 3,321,392, led Sept. 7, 1962. Incells of this type, current collectors extending into the aluminum metallayer of the cell are made of substances such as graphite or theborides, nitrides and carbides of elements of Groups 4, and 6 of thePeriodic System, particularly zirconium and titanium, for example,titanium diboride.

In the construction of aluminum reduction cells it is the practice toprovide a powdered insulating -layer between the cathode and the shellof the cell. The insulating powder underlies the molten metal pad andsurrounds the cathode contacts, whether they be of graphite or titaniumboride. When the cell is in operation a glazed layer forms on the uppersur-faceof the insulating material, adjacent the molten metal pad, andprovides a boundary between the molten metal and the bulk of theinsulating powder.

Whether the cathode collectors are made of graphite or of titaniumboride, it is of importance to match the thermal expansion of theinsulating layer of the cell as closely as possible to the thermalexpansion of the cathode collector in order to minimize thermal stresseswhich may tend to produce failure of the cathode collectors.

It has been found that the thermal expansion characteristics of the cellinsulation material can be tailored to correspond closely with those ofthe cathode collector material. This is accomplished, in accordanceIwith theV invention, by incorporating in the powdered alumina usedSAS'LISS Patented `luly 22, 1969 for the cell insulating material aminor proportion of a lluoride of aluminum.

The iluoride of aluminum may ibe aluminum fluoride, AlF3, or it may be adouble iluoride of aluminum and an alkali metal, such as cryoliteNaFAlF. These iiuorides are intimately admixed with powdered alumina ina proportion ranging Ifrom about 0.1% to about by weight. In proportionto the amount of fluoride added, it has been fund, surprisingly andunexpectedly, that the thermal expansion of the powdered alumina may becontrolled, with the result that it may be either diminished orincreased. Thus, for example, the addition of from about 0.1% to `about5% of cryolite by weight to alumina powder brings about a decreasedthermal expansion, whereas the addition of from 10% to 20% produces anincrease in thermal expansion.

The effect of additions of cryolite to alumina in diminishing orincreasing its linear thermal expansion may rbe seen from the curves ofthe accompanying drawing. The relationship of linear expansion totemperature for pure alumina is shown in the broken line curve. Theaddition of 5% of cryolite is reflected in the group of curves lyingbelow the pure alumina curve, `while the addition of larger amounts ofcryolite elevates the thermal expansion curve.

The curves of the drawing are lbased upon experimental data obtained bymeasuring thermal expansion of compacted alumina powder at temperaturesto 1000 C. in a silica dilatometer. The dilatometer was constructed bymodifying a porcelain lter crucible (Selas No. 6010) by installation ofa thermocouple well through its bottom and extending into the Crucibleapproximately one inch,

v the Well being cemented into place. The powdered material to be testedfor thermal expansion was placed in the test crucible and packed bytapping the bottom. The crucible containing the sample was placed withinthe cavity of a wire-wound silica dilatometer furnace and heated to 1000C. at the rate of about 2.8 C. per minute. The linear expansion of thesample was measured to the nearest 0.0001 inch by means of a dialindicator, the expansion being transferred to the dial lby means of afused silica rod placed on top of the sample within the dilatometerfurnace.

In accordance with the novel method of the invention, the admixing ofthe fluoride with the alumina powder is performed in any suitable typeof mixing apparatus, the mixing being carried on until a uniform mixtureis obtained.

The practice of the invention is illustrated by the yfo-llowing example,which is not, however to be regarded as limiting:

Example Where -an aluminum reduction cell is constructed so as toinclude bottom entry graphite cathode collectors, in the mannerillustrated, for example, in FIG. `6 of Patent 3,093,570, the thermalexpansion of the alumina insulating layer may be controlled to matchthat of the graphite, which is substantially less than for powderedalumina. The mean temperatures of the graphite and the alumina liningwill depend upon the cell design. However, the thermal conductivity ofthe graphite is about 1000 times that of the powdered alumina. Inpractice, the graphite temperature is about 900 C. A reasonable meantemperature for the alumina may be assumed to lbe `about 550 C. Thejunction of the glazed layer previously men- -tioned and the graphitecollector is rbelow the top of the graphite collector, the relativeposition depending upon the design and operation of the cell. Thelongitudinal thermal expansion of graphite at 900 C. is about 0.3%(Carbon Products Handbook p. 19, 1964 Ed., Union Carbide Corp., NewYork). Assuming that about of the graphite member is embedded in thealumina layer, the

problem is one of employing a refractory mixture that has the sameaverage thermal expansion at 550 C. over its length as the graphitemember has over its length, i.e. about 0.225%.

In 4accordance with the present invention, it has been found that amixture of 99% alumina and 1% cryolite has an expansion coecient of0.21%. Thus, for the design in question, a mixture of 99 parts by weightalumina to l part by Weight of cryolite `will prevent tension stressesin the graphite, providing an expansion of the alumina powder which iscompatible with the graphite collector.

Employing similar considerations, the thermal expansion of the aluminainsulating layer may be controlled to match that of a titanium diboridecathode collector.

There may also tbe employed a suitable mixture of powdered alumina andaluminum fluoride as a bed material for a reduction cell.

It will be appreciated, furthermore, that the insulating or bed layermay be subdivided into two or more layers, each adjusted by addition ofcryolite or aluminum fluoride so as to provide suitable expansioncharacteristics for a particular area.

What is claimed is:

1. An yelectrolytic reduction cell having an interior lining consistingessentially of a powdered mixture of alumina and from about 0.1% toabout 20% by weight of `a fluoride of aluminum, with a conductivecathode extending at least partially therethrough for Contact with themolten contents of the cell, said lining having a thermal expansionsubstantially similar to that of said cathode.

2. An electrolytic reduction cell having an interior -lining consistingessentially of a powdered mixture of alumina and from about 0.1% toabout 5% by weight of cryolite, with a graphite cathode extending atleast partially therethrough, -said lining having a thermal expansionsubstantially similar to that of said graphite cathode.

3. An electrolytic reduction cell having an interior lining consistingessentially of a powdered mixture of alumina and from about 10% to about20% by weight of cryolite, with `a titanium diboride cathode extendingat least partially therethrough, said lining having a thermal expansionsubstantially similar to that of said titanium diboride cathode.

4. The cell of claim 1 in which the iiuoride of aluminum is cryolite.

5. The cell of claim -1 in which the fluoride of aluminum is aluminumtriiluoride.

6. Method of lining an electrolytic reduction cell, which consistsessentially of intimately admixing powdered alumina with from vabout0.1% to about 20% by weight of a uoride of aluminum and then tampingsaid powdered mixture into the bottom of the cell so that a cathode ofconductive material extends at least partially herethrough, said mixturehaving a thermal expansion substantially similar to that of said`conductive material.

7. The method of claim 6 in `which the conductive cathode material isgraphite.

8. The method of claim 6 in which the conductive cathode material istitanium diboride.

9. The method of claim 6 in which the fluoride of aluminum is cryolite.

References Cited UNITED STATES PATENTS 3,028,324 4/ 1962 Ransley 204-2433,093,570 6/1963 Dewey 204-243 3,261,699 7/1966 Henry 204-243 3,267,1838/1966 Feinleib 204-243 JOHN -H. MACK, Primary Examiner D. R. VALENTINE,Assistant Examiner

